One technique for extracting metal from ores and other mineral material is to heap leach the material. An engineered pile, or heap, of particulate material is constructed, typically over an engineered liner and liquid collection system. Leach solution is applied to and percolated through the heap to contact the material and dissolve one or more metal of interest into the leach solution. Pregnant leach solution is collected through the collection system and is then processed to recover the dissolved metal.
A common problem with heap leaching is nonuniform leaching of metals from the heap. Even after extensive leaching, some portions of the heap often remain under-leached or even substantially un-leached. For example, leach solution often does not uniformly contact all portions of the heap because of permeability variations that exist within the heap. Such permeability variations may result in preferential flow of leach solution through higher permeability portions of the heap, leaving lower permeability portions under-leached or un-leached. As another example, the chemical properties in some portions of the heap may be less conducive to dissolution of the metal into the leach solution. For example, when heap leaching gold with cyanide leach solution under alkaline conditions, low pH spots within the heap may not respond well to the alkaline leach solution, leaving those portions under-leached or un-leached.
Metals remaining in under-leached and un-leached portions of a heap following heap leach operations often represent a significant loss.